Composite articles

ABSTRACT

There is provided a composite article which comprises a first substrate comprising a body of metal containing aluminium or galvanised steel, a second substrate and, interposed between said first and second substrates whereby to provide adhesive bonding between said substrates, an adhesive and 2 1 ,3,4 1 ,5,7-pentahydroxyflavone with the proviso that when either substrate is etched or anodised aluminium, the pentahydroxyflavone is not present thereon as a primer coat.

This application claims benefit of International application PCT/ GB94/02750, filed Dec. 16, 1994.

This invention relates to composite articles and more particularly tocomposite articles comprising at least two substrates bonded together bymeans of an intermediate layer of adhesive.

The adhesive bonding of substrates to form composite articles is atechnology of increasing importance. By appropriate selection ofadhesive, all solid substrates can be bonded although pretreatment ofthe substrate surfaces may be necessary in some cases.

Substrates which are commonly given a pretreatment include metals, themost frequently bonded metals being aluminium and steel. Thepretreatments developed for metals generally have the effect not only ofremoving surface contaminants, for example lubricants, and oxide layerswhich may be intrinsically weak or may adhere poorly to the underlyingmetal, leading to weak joints in either case but also of providing a newoxide layer which is strong and which firmly adheres to the underlyingmetal. Established pretreatments include etching for steel and etchingand/or anodising for aluminium.

In addition to the difficulties that can be experienced in attaining anacceptable initial bond strength, a significant problem associated withthe use of adhesives on metal surfaces is the deterioration in bondstrength that can occur over a period of time. This deterioration isexacerbated by high humidity and high temperatures.

Another method of improving the durability of adhesive bonded metaljoints is to apply a primer to the metal surface and a number of primershave been proposed. In Adhesives Age, June 1977, pp 30-34, Brockmanndescribes the application of 8-hydroxyquinoline and 2',3,4',5,7-pentahydroxyflavone as primers on aluminium that has been eitheretched or etched and anodised and reports their effect on the ageingbehaviour of bonds provided by phenolic and nitrile-epoxy adhesives. Inall cases, it is said, the additional application of the primerincreases the water-stability of the adhesive bond above that obtainedby etching and anodising alone.

It has now been found that a similar improvement in the stability of theadhesive bond can be achieved when 2',3,4',5,7-pentahydroxyflavone isapplied as a primer to aluminium that has been neither etched noranodised and this is not only economically advantageous but alsosurprising in view of the difficulties hitherto experienced in achievingacceptable bond strength between adhesive compositions and untreatedaluminium. Galvanised steel can also be successfully treated.Furthermore, it has been found that similar effects can be obtained bymerely incorporating pentahydroxyflavone as an additive in an adhesiveresin, thus avoiding the extra step of applying it as a primer.

Thus, according to the invention, there is provided a composite articlewhich comprises a first substrate comprising a body of metal containingaluminium or galvanised steel, a second substrate and, interposedbetween said first and second substrates whereby to provide adhesivebonding between said substrates, an adhesive and 2',3,4',5,7-pentahydroxyflavone with the proviso that when either substrate isetched or anodised aluminium, the pentahydroxyflavone is not presentthereon as a primer coat.

The first and second substrates may have any shape or configurationprovided that each has at least one surface suitable for beingadhesively bonded to a surface of the other.

The metal constituting the first substrate may be aluminium in any ofits known forms or galvanised steel in any of its known forms or analloy of either metal. Suitable forms of aluminium include barealuminium (having had no surface treatment), clad aluminium or aluminiumthat has been etched and/or anodised so long as the above proviso isobserved. It is preferred, however, to avoid the use of etched oranodised aluminium because of the environmental hazards presented byetching and anodising processes. Suitable forms of galvanised steelinclude hot dipped or electrogalvanised grades.

The second substrate may comprise any solid material. In many cases, thesecond substrate will be substantially identical in composition with thefirst substrate, the composite article of the invention comprising, forexample, two adhesively bonded sheets of aluminium or two adhesivelybonded sheets of galvanised steel. In other cases, the second substratemay be a metal other than aluminium or steel or a non-metallic inorganicmaterial, for example a ceramic material or it may be an organicmaterial, for example a polymeric material.

Adhesives which may be present in the composite articles of theinvention include known adhesive compositions such as have beendescribed, for example, at pages 554-574 of Volume 1 of theEncyclopaedia of Polymer Science and Engineering, Second Edition (JohnWiley and Sons 1985). Preferred adhesives are based on synthetic resincompositions, for example epoxy and polyurethane adhesive compositionssuch as have been fully described in the prior art.

The 2',3,4',5,7-pentahydroxyflavone (hereinafter referred to as PHF) maybe present in the adhesive itself, preferably being uniformlydistributed throughout a layer of adhesive, or, except where the metalis etched or anodised aluminium, it may be present on the surface of themetal adjacent to the adhesive as a primer coat. Effective amounts ofPHF, when incorporated in an adhesive composition, are generally withinthe range from 0.01 to 5.0% by weight based on the weight of adhesive.

The composite articles of the invention may be prepared by interposingan adhesive and PHF between the first and second substrates and curingsaid adhesive in contact with at least part of a surface of eachsubstrate provided that when either substrate is etched or anodisedaluminium, the PHF is not applied thereto as a primer coat.

A preferred method of making the composite articles of the inventioncomprises applying an adhesive and PHF to at least part of a surface ofa body of metal comprising aluminium or galvanised steel, bringing atleast part of a surface of a second substrate into contact with saidadhesive and curing the adhesive provided that when the metal is etchedor anodised aluminium, the PHF is not applied thereto as a primer coat.

In one embodiment, the method of making the composite articles of theinvention comprises applying an adhesive composition comprising anadhesive and from 0.01 to 5.0% by weight, based on the weight of theadhesive, of PHF to at least part of a surface of a body of metalcomprising aluminium or galvanised steel, bringing at least part of asurface of a second substrate into contact with said adhesivecomposition and curing the adhesive composition.

In a further embodiment, the method of making the composite articles ofthe invention comprises applying a PHF primer coat to at least part of asurface of a body of metal comprising aluminium, other than etched oranodised aluminium, or galvanised steel, applying an adhesive to theprimer coated surface, bringing at least part of a surface of a secondsubstrate into contact with said adhesive and curing the adhesive.

Before applying a primer coat to or contacting an adhesive with a metalsurface in making the composite articles of the invention, such surfacewill preferably be subjected to a conventional degreasing treatment, forexample by treating said metal surface with the vapour of a halogenatedhydrocarbon.

When the PHF is applied as a primer to a surface of the first substrate,it may be applied as a solution in a solvent such as ethanol.Application of the solution may be by spraying, dipping or brushing.Where the second substrate also comprises a body of metal such asaluminium or galvanised steel, a primer coat, when applied to the firstsubstrate, will generally also be applied to the second substrate,except when that second substrate is etched or anodised aluminium.

Application of the adhesive may be effected in a conventional mannerusing, for example, a brushing, spraying or coating technique. Preferredadhesives include polyurethane and epoxy adhesives of the typesdescribed in the prior art, for example in the aforementionedEncyclopaedia of Polymer Science and Engineering. Such adhesives maycontain conventional auxiliary agents such as catalysts, hardeners,fillers and the like. Curing of the adhesive may be effected in aconventional manner, for example by the application of elevatedtemperatures, and pressure may be applied to the substrates as necessaryto facilitate the production of a strong adhesive bond.

The composite articles of the invention may be used on structuralcomponents in the constructional and transportation industries.

The invention is illustrated but not limited by the following Examplesin which all metal substrates were first degreased in1,1,1-trichloroethane vapour before further treatment. All bond shearstrengths are the average of 4-5 replicates.

EXAMPLE 1

Adhesive joints were prepared by the application of an epoxy resinadhesive (Araldite 2007 ex Ciba-Geigy) to aluminium panels (2024 T3 cladaluminium, 25×102×1.5 mm, ex Q-Panel, Bolton, U.K.) that had beenpretreated with PHF.

In one case, an aluminium panel was immersed in a 0.1% ethanol solutionof PHF at 50° C. for 3 hours and then rinsed with clean ethanol andallowed to air dry.

In another case, a 0.01% ethanol solution of PHF was applied to thealuminium surface from a pipette (16 μl of solution applied to an areaof 25 mm×25 mm, two coats applied) and allowed to dry at roomtemperature. Other panels were treated in the same way using 0.1% and0.5% solutions of PHF in ethanol.

The joint had a bond overlap of 12.5 mm and a thickness of 0.2 mmachieved by sprinkling 0.2 mm Ballotini beads on to the adhesive beforemaking the joint. Pressure was maintained during the cure schedule (160°C./1 hour) with the aid of a bulldog clip.

The shear strengths, initially and after immersion in distilled water at50° C., are given in the following Table. A Monsanto T20 Tensometer wasused for all test

    ______________________________________              Shear strength (MPa) and locus of failure              Time immersed in water at 50° C. (hour)    Surface Pretreatment                0       240    500  1500 3000    ______________________________________    None        39.7    28.0   26.7 25.4 10.8 A    PHF (immersion)                42.6    29.1   32.2 32.2 26.1 C = A    PHF (solution 0.01%)                41.2    29.9   27.5 26.3 14.8 A > > C    PHF (solution 0.1%)                39.4    31.1   32.5 31.9 22.5 C = A    PHF (solution 0.5%)                40.7    34.6   32.5 32.4 25.0 C = A    ______________________________________     A = adhesive failure     C = cohesive failure

EXAMPLE 2

Adhesive joints were prepared by the application of a polyurethaneadhesive (Tivo 9700/5, a reactive hot melt adhesive ex Tivoli Werke,Hamburg) to aluminium panels (described in Example 1) that had beenpretreated by immersion in a 0.1% ethanolic solution of PHF as describedin Example 1.

A bond thickness of 2 mm was achieved using PTFE spacers and the bondlength was approximately 25 mm. Each bond was measured after testing todetermine the bond length accurately. Pressure was maintained during thecure schedule (180° C./30 minutes) with the aid of a bulldog clip.

The shear strengths, initially and after holding for specified times ina salt spray cabinet (5% NaCl solution/35° C.), are given in thefollowing Table.

    ______________________________________                 Shear strength (MPa) and                 locus of failure                 Time in salt spray (hour)    Surface Pretreatment                   0          500    1000    ______________________________________    None           4.8 C      0.2 A  --    PHF (solution 0.1%)                   4.7 C      4.1 C  3.7 C    ______________________________________

EXAMPLE 3

Adhesive joints were prepared by the application of a polyurethaneadhesive (Tivo 9700/5, described in Example 2) containing 1% by weightof PHF to various substrates (identified below). Bonds and curing wereas described in Example 2.

The shear strengths, initially and after immersion in distilled water at50° C., are given in the following Table.

    ______________________________________                  Shear strength (MPa) and locus of failure    Adhesion      Immersion Time (hour) in water at 50° C.    Promoter            Substrate 0            1000    ______________________________________    None    FD        3.1          2.2 A            ELO       2.7          2.1 A            Al        3.2          2.7 C    PHF     FD        3.1          2.8 C            ELO       3.6          3.2 C            Al        3.8          3.5 C    ______________________________________     FD = hot dipped galvanised steel (St 05Z 140 MC, 25 × 100 ×     1.05 mm, ex Thyssen Stahl AG)     ELO = electrogalvanised steel (St 14ZE 100/100 05 OL, 25 × 100     × 1.0 mm, ex Hoesch Stahl AG)     Al = 2024 T3 clad aluminum described in Example 1

EXAMPLE 4

Adhesive formulations were prepared having the compositions detailed inthe following Table.

    ______________________________________               1    2      3       4    5    6    ______________________________________    1.Epoxy Adduct                 29.12  29.12  29.12 29.12                                          29.12                                               29.12    2.Winnofil SPT ex ICI                 35.40  35.70  35.80 35.90                                          --   --    Resins    3.Dicy MF ex Anchor                 3.48   3.48   3.48  3.48 3.48 3.48    4.Curazol 2MZ Azine                 0.88   0.88   0.88  0.88 0.88 0.88    ex Anchor    5.Zinc Oxide ex                 1.00   1.00   1.00  1.00 --   --    Durham Chemicals    6.PHF Hydrate ex                 0.50   0.20   0.10  --   --   0.63    Aldrich    7.Epoxy Adduct                 29.12  29.12  29.12 29.12                                          29.12                                               29.12    8.Wacker HDK N20 ex                 0.50   0.50   0.50  0.50 --   --    Wacker Chemie    ______________________________________

Before use, the PHF was dried overnight in an oven at 105° C., thenground and passed through a 53 μ sieve.

The formulations were prepared by first mixing items and 1 and 2 at 600mm Hg for 10 minutes and, after stopping the stirrer and releasing thevacuum, adding items 3-6. Mixing was then continued for 1 hours at 600mm Hg and, after stopping the stirrer and release the vacuum, items 7and 8 were added. Mixing was continued for a further 15 minutes at 600mm Hg.

The epoxy adduct was prepared by stirring together 270 g of an epoxyresin (DER 331, ex Dow), 500 g of a second epoxy resin (DER 351, ex Dow)and 230 g of a polyether diamine (Jeffamine D2000 ex Texaco) for 4 hoursat 50° C.

Winnofil SPT is calcium carbonate

Dicy MF is dicyandiamide (curing agent)

Curazol 2MZ is an imidazole triazine derivative (accelerator)

Wacker HDK N20 is fumed silica

Shear strengths of joints made from the above adhesive formulations,initially and after immersion in distilled water at 50° C., are given inthe following Table.

    ______________________________________             Shear strength (MPa) and locus of failure             Time immersed in water at 50° C. (hour)    Formulation               0        500         1000    ______________________________________    1          20.00  A     13.84 C + A                                      13.17                                           A + C    2          23.17  C     17.25 C + A                                      15.21                                           A + C    3          22.75  C     17.74 C + A                                      14.72                                           A + C    4          22.08  C     15.57 C + A                                      14.20                                           A + C    5          14.0   C     --        4.8  A    6          14.8   C     --        7.0  A + C    ______________________________________

We claim:
 1. a composite article which comprises a first substratecomprising a body of metal containing aluminium or galvanised steel, asecond substrates and, interposed between said first and secondsubstrates whereby to provide adhesive bonding between said substrates,an adhesive having 2¹, 3,4¹, 5,7-pentahydroxyflavone incorporatedtherein with the proviso that neither substrate is etched or anodisedaluminium.
 2. A composite article according to claim 1 wherein thesecond substrate is substantially indentical in composition with thefirst substrate.
 3. A composite article according to claim 1 or claim 2wherein the adhesive comprises a synthetic resin composition.
 4. Acomposite article according to claim 3 wherein the synthetic resincomposition comprises an epoxy or polyurethane adhesive composition. 5.A method of making a composite article as defined in claim 1 whichcomprises interposing an adhesive having 2¹, 3,4¹,5,7-pentahydroxyflavone incorporated therein between the first andsecond substrates and curing said adhesive which is in contact with atleast part of a surface of each substrate provided that neithersubstrate is etched or anodised aluminium.
 6. A method according toclaim 5 which comprises applying an adhesive having 2¹, 3,4¹,5,7-pentahydroxyflavone incorporated therein to at least part of asurface of a body of metal comprising aluminium or galvanised steel,bringing at least part of a surface of a second substrate into contactwith said adhesive and curing the adhesive provided that neithersubstrate is etched or anodised aluminium.
 7. A method according toclaim 6 which comprises applying an adhesive composition comprising anadhesive having incorporated therein from 0.01 to 5% by weight, based onthe weight of the adhesive, of 2¹, 3,4¹, 5,7-pentahydroxyflavone to atleast part of a surface of a body of metal comprising aluminium orgalvanised steel, bringing at least part of a surface of a secondsubstrate into contact with said adhesive composition and curing theadhesive composition.
 8. A method of preparing a composite article whichcomprises a first substrate of aluminium or galvanised steel, a secondsubstrate and interposed between said first and second substrateswhereby to provide adhesive bonding between said substrates, an adhesiveand 2¹, 3,4¹, 5,7-pentahydroxyflavone, said method comprising applying aprimer coat comprising said 2¹, 3,4¹, 5,7-pentahydroxyflavone to atleast part of a surface of said first substrate, applying said adhesiveto the primer coated surface, bringing at least part of a surface ofsaid second substrate into contact with said adhesive and curing theadhesive, said first and second substrates being other than etched oranodised aluminium.
 9. A method according to claim 5 or claim 8 whereinthe second substrate comprises a body of metal containing aluminium orgalvanised steel other than etched or anodised aluminium.
 10. Acomposite article according to claim 1 or claim 4 wherein the adhesiveresin composition includes from 0.01 to 5.0% by weight of the flavonebased on the weight of the adhesive resin composition.
 11. A compositearticle according to claim 10 wherein the first substrate comprisesgalvanised steel.
 12. A method according to claim 9 wherein the primercoat comprises from 0.01 to 5.0% by weight of the flavone based on theweight of the said primer coat.